1. Field of the Invention
This invention relates to an image formation apparatus such as a copier or a printer and in particular to improvements in an image formation apparatus of the type comprising a charger having a charging member in contact with or brought close to the top of an photoreceptor and a charger used with the image formation apparatus.
2. Description of the Related Art
In recent years, demands for miniaturizing a color image formation apparatus and devises thereof have been made as the demands of the market.
For example, a tandem image formation apparatus comprises a plurality of photoreceptors such as photoconductor drums and devices such as a charger and a developing device disposed on each of the photoreceptors. As the apparatus itself is miniaturized, inevitably the developing device and the charger are placed close to each other and a new technical problem of interference between the devices, which has been no particular problem so far, is found.
To miniaturize the apparatus itself, a so-called cleanerless system wherein a cleaning device and a remaining toner collection device are not provided for each photoreceptor is proposed.
However, in this kind of cleanerless system, after transfer, remaining toner exists on the photoreceptor surface although a trace quantity of toner remains, and the remaining toner becomes xe2x80x9cmemoryxe2x80x9d at the next image formation time, adversely affecting the image quality.
Thus, in a related art, for example, an art has been proposed wherein a memory removal member (for example, a brush roll) is placed in the upstream of a charger member (for example, a charge brush) as a charger to disturb the remaining toner (for example, refer to JP-A-Hei.4-371975).
In this kind of related art, a technique of applying a bias to the memory removal member to remove the remaining toner easily is disclosed.
When a toner is accumulated in the memory removal member with the long-term use, it becomes difficult to accomplish the original object and thus, for example, a technique of temporarily holding the remaining toner and discharging and collecting the remaining toner at a predetermined timing has been already proposed (for example, refer to JP-A-Hei.11-249452).
An image formation apparatus in a related art uses, for example, a charger of a charging roll type.
As this kind of charger, for example, a charger comprising a sponge-like conductive elastic body placed on a metal shaft and coated on a surface with a fluorine resin film (PVdF) has been already proposed.
As a technical problem of this kind of image formation apparatus (charger), for example, using a charger of a charging roll type, a phenomenon in which random spots are produced at arbitrary points on paper or continuous-points or spots are produced every rotation period of a charging roll or a photoconductor drum (P/R), for example, is observed, as shown in FIG. 19.
A similar phenomenon is also observed if a charger of a charging roll type is used or in an image formation apparatus adopting a developing device of a dual-component developing type, a charger comprising a brush roll placed upstream from a charging member (for example, a charging roll) is used.
The spots are roughly classified into background spots (BKG spots) occurring in a background and image part spots occurring in an image part (for example, a halftone image), as shown in FIG. 19.
Next, the production principle of such spots is estimated. For example, as shown in FIG. 20, when a foreign substance 502 is deposited on a photoconductor drum 510 and enters a nip area between a charging roll 511 and the photoconductor drum 510, the foreign substance 502 portion shields an electric field and a tenting part is formed on a surface layer film portion of the charging roll 511 in which the foreign substance 502 intervenes so that a charge failure is caused in a part corresponding to the photoconductor drum 510 portion.
At this time, if the charge failure part caused by the foreign substance 502 shifts to the downstream of the photoconductor drum 510 and an electrostatic latent image is formed in the charge failure part and is developed, a spot having a comparatively large diameter is produced.
On the other hand, if the foreign substance 502 is deposited on the charging roll 511 or the photoconductor drum 510, a continuous point is produced every rotation period of the charging roll 511 or the photoconductor drum 510.
Since the surface layer film of the charging roll 511 uses a rigid fluorine resin film (PVdF) having a Young""s modulus of 2 GPa, the contact property between the charging roll 511 and the photoconductor drum 510 surface is poor and the discharge gap on a prenip side is unstable and becomes large in curvature and a so-called charge ghost that the previous latent image history remains because of abnormal discharge occurs.
Further pursuing the production cause of such spots, we have determined that the main cause of producing the spots is the fact that as the developing device and the charger are placed close to each other, the magnetic force from the developing device affects a shaft (usually, using a magnetic material) of the charging roll and a magnetic material of carrier, etc., is easily deposited on the surface of the charging roll because of magnetic field interference between the developing device and the shaft.
In the form in which the brush roll is placed in the upstream of the charging member (for example, a charging roll), toner and a developer (carrier) accumulate on the brush roll and drop as an aggregate on the surface of an photoreceptor as the brush roll rotates.
Particularly, the carrier is large as it has a particle diameter of 40 to 50 xcexcm and if charges, etc., are poured into the carrier by the bias applied to the brush roll, the polarity changes and the carrier easily aggregates and drops onto the photoreceptor as a coarse lump. If the carrier lump enters the charging member, the contact between the charging member and the photoreceptor surface becomes nonuniform to cause a partial charge failure to occur so that the image quality is widely affected. Specifically, a technical problem leading to the image quality defect like spots described above arises.
The carrier of the developer constantly leaks to the photoreceptor surface little by little in the developing section and easily flows out under a low-temperature, low-humidity environment or over time.
As means for handling the carrier lump, an art wherein a brush roll is provided with a flicker (a member for flicking a developer) for scrubbing away the developer deposited on the brush roll is disclosed (for example, refer to JP-A-Hei.9-54480).
However, in this technique, the possibility that the developer will scatter in the main unit of the apparatus is high and a mechanism for collecting the scrubbed developer needs to be provided; the technique is not a preferred solution from the points of mounting costs and an increase in space.
The invention is intended for solving the above-described technical problems and it is an object of the invention to provide an image formation apparatus intended for effectively avoiding a detrimental effect caused by magnetic field interference between a developing device and a charger and a charger used with the image formation apparatus.
It is another object of the invention to provide an image formation apparatus for making is possible to well avoid an image quality defect like spots according to a simple configuration and a charger used with the image formation apparatus.
It is still another object of the invention to provide an image formation apparatus for making is possible to effectively prevent an image quality defect like spots and a charge ghost from occurring as a surface layer film material of a charging member is optimized and a charger used with the image formation apparatus.
According to the invention, there is provided an image formation apparatus comprising:
a photoreceptor;
a charger having a charging member for charging the photoreceptor;
a latent image write unit for writing an electrostatic latent image onto the photoreceptor charged by the charger; and
a developing device having a developer support including a magnetic field production member, the developing device for rendering visible the electrostatic latent image written by the latent image write unit with a developer,
wherein the charging member of the charger is disposed under effect of a magnetic field produced by the magnetic field production member of the developing device; and
the charging member is made of a nonmagnetic material.